1. Field of Invention
The invention relates to a method and system for tracking satellites. More particularly, the invention relates to a method and system for tracking satellites so that unknown transmitters can be accurately located.
2. Description of Related Art
Various techniques have been proposed for locating an unknown transmitter that causes interference at a satellite receiver. One such technique is described in U.S. Pat. No. 5,008,679, entitled "Method and System for Locating an Unknown Transmitter" and issued to Effland et al. on Apr. 16, 1991, the contents of which are hereby incorporated by reference.
As described in the '679 patent, interfering signals from an unknown transmitter are received by two remote receivers (e.g., a satellite experiencing interference and an adjacent satellite) at known positions and traveling with known velocities. The remote receivers (e.g., the satellites) then retransmit the interfering signals to two ground receivers. These satellites may be equipped with a "bent pipe" transponder, which receives and retransmits a signal with no processing other than filtering, amplifying, and possibly shifting the RF frequency. Using the retransmitted signals received at the ground receivers, a processor calculates the differences in time and frequency of arrival of the signals from the two remote receivers, referred to as "TDOA" and "FDOA," respectively, to determine the location of the unknown transmitter.
To achieve a reasonable degree of accuracy using this technique, it is important that the position and velocity of each of the remote receivers (e.g., the satellites) are known. Slight inaccuracies in the position or velocity of the satellites can result in significant error in relating the measurements of TDOA and FDOA to a position on the ground.
Usually, these satellites are placed in geostationary orbits; that is, their orbits are chosen so they have a nearly constant apparent position relative to a user on Earth. To counteract forces from, for example, the gravity of the sun and moon, a satellite has thrusters used to alter its velocity and maintain it in geostationary orbit within a desired distance, typically .+-.0.025 degrees as viewed from Earth, of its assigned position. Proper orbital correction, however, requires that the satellite orbit prior to correction be known. Thus, it is desirable to determine accurately the orbit of the satellite in order to maintain geostationary orbit.
Satellite orbits are typically defined by six numbers that specify a path traced out by the orbiting satellite and the position of the satellite along that path at a particular time, referred to as the "epoch time." These six numbers may be represented by three Cartesian coordinates specifying the satellite's position and three Cartesian velocity components specifying the satellite's velocity at the epoch time. Alternatively, these numbers may be represented in accordance with other conventional techniques, such as the "classical orbital elements."
One prior art technique of determining the positions and velocities of satellites uses tone ranging to measure the distance from a ranging station to the satellite. Typically, a ranging station transmits two or more unmodulated (CW) signals to a satellite, which retransmits the signals back to the station. The difference in roundtrip phase delay among the signals is used to determine the satellite's distance from the ranging station at a particular time. Repeated distance measurements from one or more ranging stations are analyzed to determine satellite orbits.
For example, if a satellite is at a distance d from a ranging station, the roundtrip phase shift of a signal transmitted from the station at frequency .omega. will be: ##EQU1## where d is the distance from station to satellite, .phi..sub.sat is a phase shift introduced by the satellite's frequency translation equipment, .OMEGA. is the transponder design frequency translation, and c is the speed of light. Small effects influenced by, for example, the propagation medium and the relative motion of station and satellite are omitted for simplicity.
From this relation, it is simple to obtain the derivative of equation (1) as follows: ##EQU2## The derivative .differential..phi./.differential..omega. can be estimated, for example, by measuring .phi. at two frequencies and dividing by the difference in frequency. Using this estimate, the distance d can be calculated from equation (2).
In practice, it is desirable to measure the phase at a plurality of frequencies with different frequency separations to permit d to be determined unambiguously even though the phase can only be determined modulo 2.pi.. Yet, it is also desirable to use the tone ranging technique without interfering with the normal use of the satellite. Because of these desires, however, the tone ranging activity is restricted to a relatively narrow total bandwidth, reducing the potential accuracy of an individual measurement. The accuracy of orbit determination is directly related to the accuracy of the individual ranging measurements. Thus, it is desirable to determine more accurately the position and velocity of a satellite without interfering with its normal use, in order to determine the orbital parameter more accurately.
Moreover, prior art satellite tracking techniques require the attention of many persons for their operation and often produce data that are ambiguous as a result of imprecision in the measurement of the phase for a particular tone. Thus, it is also desirable to track satellite orbits accurately with minimal effort.
In addition, prior art systems require a multiplicity of measurements of d (or equivalently the time delay d/c) to be measured at different times and from different ranging stations in order to determine the orbital parameters. Usually, a number of measurements are obtained and processed to determine a satellite orbit and predict the time of a satellite maneuver. Another set of measurements is obtained and processed after the satellite maneuver to determine the new satellite orbit. This procedure must be repeated after each satellite maneuver, which may be anywhere from 4 to 24 days from the previous satellite maneuver. Performing an orbital maneuver based on an inaccurate orbit results in an inaccurate orbital correction, which may waste fuel and reduce the satellite's usable life. Thus, it is desirable to track satellite orbits continuously in order to maintain accurate determinations of the satellites' positions and velocities.